Nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, have a high energy density and are therefore in wide use as batteries for personal computers, mobile phones, portable information terminals, and the like. Such nonaqueous electrolyte secondary batteries have recently been developed as on-vehicle batteries.
As a member of such a nonaqueous electrolyte secondary battery, a separator having excellent heat resistance has been developed.
As a nonaqueous electrolyte secondary battery porous layer included in a separator for a nonaqueous electrolyte secondary battery (hereinafter referred to as “nonaqueous electrolyte secondary battery separator”) which nonaqueous electrolyte secondary battery separator has excellent heat resistance, a porous layer including an organic filler has been developed. For example, Patent Literature 1 discloses a lithium secondary battery separator including a porous base material and a coating layer which is located on at least one surface of the porous base material and which contains organic particles and a binder polymer, the binder polymer being contained in an amount of 50% by weight to 99% by weight relative to a total weight of the coating layer.